Electronic circuit for supplying a high-pressure discharge arc lamp

ABSTRACT

The invention relates to an electronic circuit and to a method of supplying a high-pressure discharge arc lamp ( 12 ). The circuit comprises a DC-AC converter, for which purpose two controllable switching elements T 1 , T 2  are connected in the form of a half bridge to an operating potential U+ and a reference potential ( 10 ). The circuit further comprises a two-stage filter arrangement. The lamp is connected to a coil Lign of the second filter stage, and the same connection terminal of this coil L ign  is connected to the reference potential ( 10 ) via a capacitor C ign . To make the circuit as small and as economical as possible, while high high-frequency interference peaks and strong currents in the circuit are avoided, it is proposed that a coil Tr filt  of the first filter stage has at least three taps. The first, outer tap is connected to the output of the half bridge, the second, central tap to the second connection terminal of the coil L ign , and the third, outer tap to the reference potential ( 10 ) via a capacitor C filt .

The invention relates to an electronic circuit for supplying ahigh-pressure discharge arc lamp and to methods of operating ahigh-pressure discharge arc lamp with such an electronic circuit.

High-pressure discharge arc lamps are used, for example, in modern dataand video projectors. They have a very high power and are characterizedby a particularly short discharge arc. It is possible on the basis ofoptical laws to manufacture projectors with such lamps and with smalloptical systems, which nevertheless have a high luminous efficacy, i.e.produce a bright image. This has led to a considerable reduction in sizeand also in cost of the projectors.

At the same time, however, new requirements have arisen as to thedimensions and cost of the electronic components in such a projector. Anessential electronic component here is the electronic supply circuit,also denoted ballast, for the high-pressure discharge arc lamp.

The supply circuit has the task first of all of generating a voltage ina range of several kilovolts for a short period for igniting the lamp,which is necessary to initiate the arc discharge. During subsequentoperation, the supply circuit has the task of controlling the currentthrough the lamp such that a constant average power adjusts itself inthe lamp. A particular feature here is that high-pressure arc dischargelamps generally have a negative current-voltage characteristic whichrequires a supply circuit capable of supplying a limited current. Thecurrent through the lamp can be kept constant with considerabledifficulties only in the case of voltage-limiting circuits. It isfurthermore usual to operate high-pressure discharge arc lamps with alow-frequency square-wave alternating current. This allows for a moreeven load on the lamp electrodes than in the case of a direct currentsupply, as well as a constant, flicker-free lamp brightness.

Various electronic circuits for supplying a high-pressure discharge arclamp are known in the art. These circuits usually comprise a DC-ACconverter bridge circuit which is supplied with a constant DC voltageand which provides a low-frequency alternating current at its output.

A supply circuit that manages to operate with a particularly smallnumber of power components is described in the publication U.S. Pat. No.6,020,691. The small number of power components is achieved in that thecircuit uses a DC-AC converter in the form of a half bridge circuit.This circuit is shown in FIG. 9.

The circuit comprises a half bridge which has a transistor Q1, Q2 ineach of its two bridge branches. To control the transistors Q1, Q2, acontrol device 91 is provided. The half bridge is connected at one sideto a DC voltage source via a terminal Vbus 92 and at the other side to areference potential 0 for the purpose of supplying the circuit. Thecontrol device 91 controls the transistors Q1, Q2 such that an ACcurrent is made available at the output of the half bridge, i.e. betweenthe two transistors Q1, Q2. Each of the transistors Q1, Q2 has a diodeD1, D2 connected in parallel thereto, which diode is conducting in adirection from the reference potential to the supply voltage. Twocapacitors Ca, Cb arranged in series are connected parallel to theentire half bridge, also between the supply voltage and the referencepotential. These capacitors Ca, Cb replace the second half bridge in theotherwise often implemented DC-AC converter comprising a full bridgecircuit.

A two-stage low-pass filter is connected between the output of the halfbridge and the junction point between the two capacitors Ca, Cb. Thefirst filter stage of the two-stage low-pass filter is designed toreduce high-frequency interferences during normal operation, whereas thesecond filter stage mainly serves to generate a high-frequency ignitionvoltage. The first filter stage for this purpose comprises a first coilL1 and a third capacitor C1, and the second filter stage comprises asecond coil L2 and a fourth capacitor C2. The first terminal of the coilL1 is connected to the output of the half bridge here. The secondterminal of the coil L1 is connected via the capacitor C1 to thejunction point between the two capacitors Ca, Cb. Furthermore, thesecond terminal of the coil L1 is connected to the first terminal of thecoil L2. The second terminal of the coil L2 is also connected to thejunction point between the two capacitors Ca, Cb, on the one hand viathe capacitor C2, and on the other hand via a series arrangement of ahigh-pressure discharge arc lamp LMP and a resistor Rs. The secondfilter stage comprising the coil L2 and the capacitor C2 preferably hasa higher resonance frequency than the first filter stage comprising thecoil L1 and the capacitor C1. The two capacitors Ca, Cb must bedimensioned sufficiently large so as to be capable of accommodating thelow-frequency component of the lamp current without too high voltagefluctuations.

A current sensor 93 senses the current between the lamp LMP and theresistor Rs and supplies it as a parameter to the control device 91.

To obtain the high voltage necessary for igniting the lamp, the resonantcircuit formed by the second coil L2 and the second capacitor C2 isexcited by a suitable control of the circuit by the control device 91.Extremely high currents arise in the circuit as a result of this, whichmay be of the order of ten times the normal lamp current, if an ignitionvoltage in the kilovolt range is to be generated. This means that thecoil L2 must be constructed such that it is not saturated at thesecurrents. If the second filter stage L2, C2 has a higher resonancefrequency than the first filter stage L1, C1, moreover, it is only thealready strongly attenuated AC voltage of the half bridge Q1, Q2 that isavailable for exciting the resonance. This attenuated AC voltagerequires a particularly high quality factor of the tuned circuit L2, C2,to which is linked a correspondingly high expenditure in providing thecomponents. Furthermore, the simultaneous requirements of a high voltageand a low AC component in the lamp during normal operation lead to theoccurrence of comparatively high currents in the circuit. Finally, thearrangement described of coils and capacitors may lead to highhigh-frequency interference peaks at least during the ignition phase.

The invention has for its object to develop the electronic circuit forsupplying a high-pressure discharge arc lamp as shown in FIG. 9 furthersuch that the described disadvantages can be avoided without detractingfrom the existing advantages. The invention in particular has for itsobject to provide as small and as economical an electronic circuit aspossible for supplying a high-pressure discharge arc lamp in which thehigh high-frequency interference peak and strong currents in the circuitare avoided.

This object is achieved by an electronic circuit as claimed in claim 1.

The invention has the particular feature that the first filter stage ofthe two-stage filter has a coil with three taps instead of a coil withtwo end terminals. The coil of the second filter stage is connected tothe central tap of the coil having the three taps, while the outerterminals of the coil are again connected to the output of the halfbridge on the one side, and on the other side to the reference potentialof the circuit via a capacitor. It is possible with such an embodimentto provide different functions of the coil for different operationalmodes of the circuit.

In principle, the combination of the coil with three taps and thecapacitor connected to this coil represents a serial tuned circuit. Ifsuch a tuned circuit is operated above its resonance frequency, thevoltage gradient across the capacitor is in counter phase to the voltagegradient at the input of the tuned circuit. The tapped coil may now beregarded as a kind of inductive voltage divider at whose central tap asuperimposed value of the voltages at the two ends can be taken off. Ifthe two voltages are in counter phase, it is achieved through a correctchoice of the ratio of the two partial windings that the two voltagescancel each other out. The arrangement of the coil with three taps andthe capacitor connected to this coil thus performs the function of ablocking filter for a certain, exactly defined blocking frequency.

It is accordingly an advantage of the invention that each and everyhigh-frequency component can be suppressed in the lamp for a selectableblocking frequency.

At the same time, the coil with three taps merely operates as a voltagedivider without blocking action for all other frequencies. If theoperational frequency, moreover, is considerably higher than theblocking frequency, there will be no strong attenuation or damping ofthe output signal of the half bridge owing to the filter action of thefirst filter stage. This makes it possible to choose the quality factorof the tuned circuit of the second filter stage to be lower than in theknown half bridge circuit, without losing the voltage increase necessaryfor ignition.

Since the combination of the coil with three taps and the connectedcapacitor has a blocking filter action, the capacitor of the firstfilter stage can be dimensioned considerably smaller than in aconventional circuit if at the same time the switching frequency of thehalf bridge is identical to the blocking frequency of the filter.

Advantageous embodiments of the invention are apparent from thedependent claims.

The dimensioning of the central tap of the coil with three taps and ofthe capacitor connected to this coil is of particular importance here.The two components are preferably dimensioned such that the frequencycomponent at the output of the half bridge, which is dominant duringnormal operation of the lamp, is extinguished at the central tap of thecoil with three taps. The voltage at the output of the half bridge doescomprise multiples of this dominant frequency which are not suppressed.However, an effective filter is available for said multiples in thefurther coil, because the interfering frequencies are higher by integermultiples. It is thus possible to obtain the particularly complicatedfiltering of the base frequency of the switch mode power supply by meansof particularly small components.

The second filter stage, however, is preferably dimensioned such thatits resonance frequency lies well above the blocking frequency of thefirst filter stage. Lamp ignition is made possible thereby duringoperation of the circuit at this frequency without the excitation signalbeing strongly damped and without an extremely high current beinggenerated through the filter components.

Further advantageous embodiments and advantages of the invention aregiven in the ensuing description of embodiments of the electroniccircuit according to the invention, with:

FIG. 1 showing a first embodiment of the electronic circuit according tothe invention,

FIG. 2 being a diagram of an embodiment of the control circuit of thecircuit of FIG. 1,

FIG. 3 showing examples of current and voltage gradients in the circuitof FIG. 1 during an ignition phase,

FIG. 4 showing examples of voltage gradients in the circuit of FIG. 1during a heating-up phase,

FIG. 5 showing examples of current gradients in the circuit of FIG. 1during normal operation,

FIG. 6 showing a second embodiment of the electronic circuit accordingto the invention,

FIG. 7 showing a third embodiment of the electronic circuit according tothe invention,

FIG. 8 showing a fourth embodiment of the electronic circuit accordingto the invention, and

FIG. 9 showing an electronic circuit for supplying high-pressuredischarge lamps from the prior art.

FIG. 1 shows a first embodiment of the electronic circuit according tothe invention.

It comprises two power transistors T_(1 and T) ₂ which are connected toa supply voltage U₊ and to the reference potential 10 of the circuit inthe manner of a half bridge. A series arrangement of two electrolyticcapacitors C_(DC2), C_(DC1) is connected in parallel to the entire halfbridge between the supply voltage U₊ and the reference potential 10 ofthe circuit. A coil Tr_(filt) with three taps is connected by its firstterminal to the output 11 of the half bridge. The central tap of thecoil Tr_(filt) is connected to the first terminal of a second coilL_(ign). The remaining third, outer tap or terminal of the coilTr_(filt) is connected via a capacitor C_(filt) directly to thereference potential 10 of the circuit. The coil Tr_(filt) and thecapacitor C_(filt) are dimensioned such that the frequency component atthe output 11 of the half bridge which is dominant during normaloperation of the circuit is extinguished at the central tap of the coilTr_(filt), i.e. it forms a blocking frequency.

The second terminal of the coil L_(ign) is connected to a furthercapacitor C_(ign) having the reference potential 10 of the circuit.Furthermore, the second terminal of the coil L_(ign) is connected to afirst terminal for a high-pressure discharge arc lamp 12. The secondterminal of the high-pressure discharge arc lamp 12 is connected to thejunction point between the two capacitors C_(DC1) and C_(DC2). The coilL_(ign) and the capacitor C_(ign) are dimensioned such that they form atuned circuit having a resonance frequency which lies above the blockingfrequency mentioned above.

A current sensor 13 which senses the current i₁ through the coilTr_(filt) is furthermore provided between the output 11 of the halfbridge and the first terminal of the coil Tr_(filt). The value measuredby the current sensor 13 is supplied to a control circuit 14 whichswitches the transistors T_(1 and T) ₂ of the half bridge on and off inalternation in dependence on the received value such that a desiredcurrent gradient is achieved in the lamp 12.

FIG. 2 shows a possible embodiment of a suitable control circuit 14 fordriving the transistors T₁, T₂ of the half bridge shown in FIG. 1.

The control circuit comprises, first of all for lamp ignition, a firstfrequency generator 211 which generates a high-frequency signal having afrequency F1 and delivers it to a multiplexer 201 via two complementaryoutputs 212, 213. The frequency F1 here corresponds substantially to theresonance frequency of the ignition circuit formed by the coil L_(ign)and the capacitor C_(ign) of the circuit shown in FIG. 1.

For normal lamp operation, moreover, the control circuit comprises asecond frequency generator 221 which generates pulses having a frequencyF2, which pulses set a flipflop 222 each time. The frequency F2 formsthe dominant frequency component at the output of the half bridge duringnormal operation of the circuit.

The measured value of the current i₁ supplied by the current sensors 13of FIG. 1 is also fed to a comparator 223, while the second input of thecomparator 223 is fed from a low-frequency waveform generator 224. Thesignal from the waveform generator 224 here represents the desired lampcurrent gradient. The output of the comparator 223 and a further signalof the waveform generator 224, this latter signal indicating theinstantaneously desired current direction in the lamp 12 as a polaritysignal, are supplied to an EXCLUSIVE-OR member 225. A desired positivelamp current leads to the generation of a high-level polarity signal “1”by the waveform generator 224 and accordingly to an inversion of thecomparator output by the EXCLUSIVE-OR member 225. The output of theEXCLUSIVE-OR member 225 is connected to a reset input of the flipflop222. A high-level output signal “1” of the EXCLUSIE-OR member 225achieves a reset of the flipflop 222 each time.

The flipflop 222 supplies two complementary output signals Q and /Q. Thetwo output signals are supplied, each via a respective EXCLUSIVE-ORmember 226, 227, also to the multiplexer 201. The second input signal ofthe two EXCLUSIVE-OR members 226, 227 again is the polarity signal ofthe waveform generator 224. A process controller 202 switches themultiplexer 201 in dependence on the measured current i₁ either to thecomplementary outputs of the second frequency generator 211 or to thecomplementary outputs of the EXCLUSIVE-OR members 226, 227. The signalpair selected at any time is then supplied by the multiplexer 201 via arespective delay stage 203, 204 to the control terminals of the powertransistors T_(1 and T) ₂.

The supply of a high-pressure discharge lamp 12 by means of the circuitshown in FIGS. 1 and 2 will now be described below.

In the non-ignited state, the high-pressure discharge lamp 12 is to beregarded as an interruption. This means that the current in the coilL_(ign) can only flow away through the capacitor C_(ign). As a result,the coil L_(ign) is supplemented by the capacitor C_(ign) so as to forma series resonant circuit. Now when the half bridge is operated at theresonance frequency of this series resonant circuit, a high voltage willbuild up in the resonant circuit L_(ign), C_(ign). If the resonancefrequency of the resonant circuit L_(ign), C_(ign) is unequal to theblocking frequency of the filter formed by the coil Tr_(fit) and thecapacitor C_(filt), an excitation of the resonant circuit L_(ign),C_(ign) can still take place because the inductive voltage dividerformed by the coil Tr_(filt) is not attuned. If the resonance frequencyof the resonant circuit L_(ign), C_(ign) is considerably higher than theblocking frequency, the voltage across the capacitor C_(filt) may beregarded as constant by approximation. The residual voltage at thecentral tap of the coil Tr_(filt) in this case corresponds to thewinding ratio of the coil Tr_(filt). This now makes it possible toutilize any desired high frequencies for exciting the ignition circuitL_(ign), C_(ign) without the excitation signal being too strongly dampedby the filter action of the first filter stage Tr_(filt), C_(filt).

When the process controller 202 recognizes from the measured values ofthe current i₁ obtained from the current sensor 13 that no low-frequencycurrent flows through the coil Tr_(filt) at the moment, it is concludedthat the lamp 12 is not operating. The process controller 202 thenswitches the complementary outputs 212, 213 of the first frequencygenerator 211 directly to the delay stages 203, 204 for the purpose ofignition of the lamp 12. The resonance frequency of the ignition circuitL_(ign), C_(ign) is excited thereby in the circuit, which in its turngenerates a sufficiently high voltage for igniting the lamp 12, of theorder of several kilovolts. At the same time, the output current i₂ ofthe half bridge remains comparatively low because of the transformerfunction of the coil Tr_(filt). The coil L_(ign) has a limiting effecton the lamp current i_(lamp) at the adjusted high resonance frequency ofthe ignition circuit L_(ign), C_(ign).

A particularly advantageous situation arises when the resonancefrequency of the ignition circuit L_(ign), C_(ign) is exactly threetimes the blocking frequency of the first filter stage Tr_(filt),C_(filt). It is possible then to excite the ignition circuit L_(ign),C_(ign) by means of the third harmonic of the square-wave gradient ofthe voltage U₁ at the output 11 of the half bridge. This results incurrent amplitudes i₁ in the components of the circuit which are nogreater than during normal operation if said circuit is optimized withthe smallest possible components for maintaining usual current waveformsduring normal operation. FIG. 3 shows a relevant square-wave gradient ofthe voltage U₁ at the output of the half bridge in volts, a relevantgradient of the voltage U_(lamp) across the lamp with the threefoldfrequency in volts, and a relevant gradient of the output current i₁ ofthe half bridge in milliamps as a function of time so as to clarify theabove.

The ignition operation should be maintained for at least one second, butpreferably at least two seconds so as to ensure that the lamp 12 willignite reliably.

Immediately after ignition, high-pressure discharge lamps require a highoperating voltage of more than 250 V for a short time until the lampelectrodes have heated up sufficiently for entering the arc mode. In thenormal case, however, the circuit described is capable of generating alamp voltage of at most half the operating voltage U₊, i.e. typically200 V for an operating voltage of at most 400 V.

A resonance effect may be utilized again for artificially raising theoperating voltage. The resonant circuit formed by the coil L_(ign), andthe capacitor C_(ign) is not eligible for this, because its loadingcapacity is insufficient if it was suitably dimensioned. The arrangementof the coil Tr_(filt) and the capacitor C_(filt), however, also forms aresonant circuit which is usually operated above its resonancefrequency.

The process controller 202 first achieves for the transition phase thatthe multiplexer 201 uses the output signals of the EXCLUSIVE-OR members226, 227 as its input signals instead of the complementary outputsignals of the first frequency generator 211. In addition, the frequencyF2 of the second frequency generator 221 is lowered in the direction ofthe resonance frequency of the resonant circuit Tr_(filt), C_(filt) bythe process controller 202. The triggering procedure of the transistorsT₁, T₂ corresponds to the triggering in normal operation to be describedfurther below during this. The reduced frequency F2 results in a voltagerise of medium frequency which generates a sufficient current throughthe lamp 12 for heating up the electrodes. At the same time, a strongrise of the lamp current is prevented by the frequency and by theinductance of the coil Tr_(filt). The lamp voltage gradient U_(lamp) andthe gradient of the output voltage of the half bridge U₁ during such aheating-up phase are plotted in FIG. 4 in volts as a function of time.

After the lamp 12 has ignited and its electrodes have becomesufficiently heated, the electronic circuit of FIG. 1 may now take overnormal operation. For this purpose, the frequency F2 of the frequencygenerator 221 is returned to its original value.

Normally, it may now first be assumed that the two capacitors C_(DC1)and C_(DC2) have become charged such that the voltage at their junctionpoint amounts to approximately half the operating voltage U₊ of thecircuit. A low-frequency alternating current is now to be generated inthe lamp 12 by means of a suitable control of the transistors T₁, T₂,often with a square-wave characteristic.

This control by the control circuit 14 will now be explained withreference to the example of the positive half wave of the lamp currenti_(lamp). The starting position assumed here is that the currenti_(lamp) and the voltage across the lamp are positive, and that thecurrent i₁ in the coil Tr_(filt) is also positive. The voltage acrossthe capacitor C_(filt) is approximately the sum of half the operatingvoltage U₊ and the positive lamp voltage. The flipflop 222 is not set.Since the polarity signal of the waveform generator 224 indicates thatthe lamp current should be instantaneously positive, the EXCLUSIVE-ORmembers 226, 227 invert the complementary outputs Q, /Q of the flipflop222. As a result, the transistor T₁ is switched on and the transistor T₂is switched off.

Now the flipflop 222 is set by a pulse from the second frequencygenerator 221. A “1” is generated thereby at the Q output of theflipflop 222, which switches off the transistor T₁ after inversion bythe associated EXCLUSIVE-OR member 226 without further delay. A “0” isgenerated at the /Q output of the flipflop 222, which switches on thetransistor T₂ after inversion and after a delay time DT has elapsed. Thedelay time DT serves to exclude that the two transistors T₁, T₂ of thehalf bridge can be conducting at the same time.

The voltage at the output 11 of the half bridge is now 0 V. This meansthat the positive voltage i₁ in the coil Tr_(filt) becomes smallerquickly because the right-hand terminal is at a high potential. As wasnoted above, this high potential amounts to approximately the sum ofhalf the operating voltage U₊ and the lamp voltage. When the referencevalue supplied by the waveform generator 224 is undershot by themeasured value of the current i₁ at the comparator 223, said comparator223 will deliver a low-level signal “0” at its output. This signal isinverted by the EXCLUSIVE-OR member 225 because of the still high-levelpolarity signal of the waveform generator 224 and resets the flipflop222. This switches off the transistor T₂ again and switches on thetransistor T₁ after a delay time DT.

The operating voltage U₊, which is higher than the voltage across thecapacitor C_(filt), is applied to the output of the half bridge 11again, so that the current i₁ in the coil Tr_(filt) rises again. Thisstate is maintained up to the next pulse of the frequency generator 221.Since no low-frequency current component can flow through the capacitorC_(filt), the low-frequency component enters the capacitors C_(DC1) andC_(DC2) via the coil L_(ign) through the lamp 12. The capacitor C_(ign)has such a small value that it is irrelevant for the lamp currentI_(lamp) when the lamp 12 has been ignited.

FIG. 5 plots the current i₁ through the coil Tr_(filt) and the lampcurrent i_(lamp) in amps over two cycles of the signal F2 of thefrequency generator 221 during the positive half wave of the lampcurrent. The broken line in addition represents a reference currenti_(ref) which is the reference value of the waveform generator 224 forthe positive half wave of the lamp current.

It is apparent that a rise in the reference value of the waveformgenerator 224 will move the entire current gradient in parallel alongwith it, i.e. also the average value of the lamp current i_(lamp) willrise to exactly the same degree. This thus provides a simple possibilityof adjusting the value of the lamp current i_(lamp). FIG. 5 also showsthat the current i₁ in the coil Tr_(filt) changes its sign also with apositive lamp current i_(lamp) in spite of a superimposed DC component.This renders it possible to use the so-termed zero-voltage switchingduring normal operation as well as during transitional operation.

Since the dominant frequency component at the output of the half bridgeis the frequency F2 corresponding to the blocking frequency, thisfrequency component is extinguished at the central tap of the coilTr_(filt). The voltage across the capacitor C_(filt) cannot now beregarded as constant any more, but it is subject to substantialfluctuations. These fluctuations are reflected in FIG. 5 as a deviationof the current gradient i_(lamp) from a constant gradient.

The lamp current i_(lamp) and the lamp voltage are negative during thenegative half wave. The voltage across the capacitor C_(filt) now is thesum of half the operating voltage U₊ and the negative value of the lampvoltage. The waveform generator 224 now supplies a “0” polarity signalin accordance with the envisaged lamp current gradient. This means thatthe EXCLUSIVE-OR members 225, 226, 227 have no influence, they merelypass on the respective signals applied to them apart from the polaritysignal at their outputs again. It is first assumed that the current i₁in the coil Tr_(filt) is negative. The pulse of the frequency generator221 again sets the flipflop 222, but now the transistor T₁ is switchedon and the transistor T₂ is switched off after a delay time DT therebythis time. The operating voltage U₊ of the circuit is subsequentlypresent at the output 11 of the half bridge. This voltage issubstantially higher than the voltage across the capacitor C_(filt), sothat the current i₁ in the coil Tr_(filt) rises quickly. When thereference value supplied by the waveform generator 224 is exceeded, thecomparator 223 generates a “1” at its output, whereby the flipflop 222is reset again. This switches off the transistor T₁ and switches on thetransistor T₂ after a delay time DT. The voltage at the output 11 of thehalf bridge then is 0 V. Since the voltage across the capacitor C_(filt)is greater than zero, the current i₁ in the coil Tr_(filt) is built upagain.

FIGS. 6 to 8 show possible modifications of the circuit of FIG. 1, whichmodified circuits, however, perform the same basic functions as thecircuit of FIG. 1. The fundamental construction is the same as in FIG. 1each time, and corresponding components have been given the samereference symbols, so that only the respective differences need bedescribed below. The control circuit not shown in FIGS. 6 to 8 may alsobe the same as the control circuit of the first embodiment.

In the first modification shown in FIG. 6, the third, outermost tap ofthe coil Tr_(filt) is additionally connected to the operating voltage U₊of the circuit via a capacitor C_(filtb). The capacitor denoted C_(filt)of FIG. 1 is denoted C_(filta) in FIG. 6 for better distinguishability.Similarly, the second terminal of the coil L_(ign) is additionallyconnected to the operating voltage U₊ of the circuit via a capacitorC_(ignb). The capacitor denoted C_(ign) in FIG. 1 is denoted C_(igna) inFIG. 6.

In the second modification shown in FIG. 7, the capacitors C_(filt) andC_(ign) are not directly connected to the reference potential of thecircuit, but are connected to the junction point between the capacitorsC_(DC1), C_(DC2). It is clear from this that the coils may also beindirectly connected to the reference potential of the circuit via therespective capacitor designed for forming a resonant circuit.

In the third modification shown in FIG. 8, an additional capacitorC_(dvdtb), C_(dvdta) is connected in parallel to each transistor T₁, T₂,respectively. The capacitors C_(dvdtb), C_(dvdta) here serve to limitthe speed of the voltage rise during switching over of the transistorsT₁, T₂ of the half bridge. Alternatively, only one of the additionalcapacitors may be used. The capacitors for limiting the speed of thevoltage rise in the embodiment of FIG. 8 lead to particularly lowswitching losses in the zero-voltage switching mentioned with referenceto FIG. 5.

The description of the manner of operation of the circuit of FIG. 1 isvalid in a corresponding manner for these as well as other modificationsof the circuit of FIG. 1 in which the capacitors are arranged in anequivalent high-frequency manner with respect to the output current ofthe half bridge and the lamp current. Similarly, some of the componentsmay be connected, for example, in series with additional resistors.

The embodiments described represent only a few examples from among aplurality of possible implementations of the invention.

1. An electronic circuit for supplying a high-pressure discharge arclamp (12), which circuit comprises a half bridge comprising at least onecontrollable switching element (T₁, T₂) in each of its bridge branchesfor providing an alternating current, and at least two coils (L_(ign),Tr_(filt)), four capacitors (C_(ign), C_(DC2), C_(DC1), C_(filt),C_(igna), C_(filta)), and two connection terminals for a high-pressuredischarge arc lamp (12), which half bridge (T₁, T₂) is connected betweena connection terminal of the circuit for providing an operatingpotential (U₊) and a connection terminal of the circuit for providing areference potential (10), while a first connection terminal of the firstcoil (L_(ign)) is connected to the first connection terminal for ahigh-pressure discharge arc lamp (12) and to the connection terminal forthe reference potential (10) at least via the first capacitor (C_(ign),C_(igna)), and the second connection terminal for a high-pressuredischarge arc lamp (12) is connected to the connection terminal for theoperating potential (U₊) at least via the second capacitor (C_(DC2)) aswell as to the connection terminal for the reference potential (10) atleast via the third capacitor (C_(DC1)), characterized in that thesecond coil (Tr_(filt)) has at least three taps, of which a first, outertap is connected to the output (11) of the half bridge (T₁, T₂), ofwhich a second, central tap is connected to the second connectionterminal of the first coil (L_(ign)), and of which a third, outer tap isconnected to the connection terminal for the reference potential (10) atleast via the fourth capacitor (C_(filt), C_(filta)).
 2. An electroniccircuit as claimed in claim 1, characterized in that the first capacitorand the fourth capacitor (C_(ign), C_(filt), C_(igna), C_(filta)) areeach directly connected to the connection terminal for the referencepotential.
 3. An electronic circuit as claimed in claim 1, characterizedin that the first capacitor and the fourth capacitor (C_(ign), C_(filt))are each connected to the connection terminal for the referencepotential via the third capacitor (C_(DC1)) and are each connected tothe connection terminal for the operating potential (U₊) via the secondcapacitor (C_(DC2)).
 4. An electronic circuit as claimed in claim 1,characterized in that the first connection terminal of the first coil(L_(ign)) is additionally connected to the connection terminal for theoperating potential (U₊) via a fifth capacitor (C_(ignb)), and/or inthat the third, outer tap of the second coil (Tr_(filt)) is additionallyconnected to the connection terminal for the operating potential (U₊)via a sixth capacitor (C_(filtb)).
 5. An electronic circuit as claimedin claim 1, characterized in that the output of the half bridge (T₁, T₂)is additionally connected to the connection terminal for the referencepotential via at least one further capacitor (C_(dvdta)).
 6. Anelectronic circuit as claimed in claim 1, characterized in that theoutput of the half bridge (T₁, T₂) is additionally connected to theconnection terminal for the operating potential (U₊) via at least onefurther capacitor (C_(dvdtb)).
 7. An electronic circuit as claimed inclaim 1, characterized in that the arrangement consisting of the secondcoil (Tr_(filt)) and the fourth capacitor (C_(filt)) forms a blockingfilter for the central tap of the second coil (Tr_(filt)) at a switchingfrequency with which the controllable switching elements (T₁, T₂) of thehalf bridge are preferably switched in normal operation.
 8. Anelectronic circuit as claimed in claim 1, characterized in that theresonance frequency of a resonant circuit comprising the first coil(L_(ign)) and the first capacitor (C_(ign)) is higher than a frequencyat which the arrangement of the second coil (Tr_(filt)) and the fourthcapacitor (C_(filt)) forms a blocking filter for the central tap of thesecond coil (Tr_(filt)).
 9. An electronic circuit as claimed in claim 8,characterized in that the resonance frequency of a resonant circuitcomprising the first coil (L_(ign)) and the first capacitor (C_(ign)) isan odd multiple of the frequency at which the arrangement of the secondcoil (Tr_(filt)) and the fourth capacitor (C_(filt)) forms a blockingfilter for the central tap of the second coil (Tr_(filt)).
 10. Anelectronic circuit as claimed in claim 1, characterized by a controlcircuit (14) for controlling the switching elements (T₁, T₂) of the halfbridge, and by a current sensor arranged between the output (11) of thehalf bridge and the second coil (Tr_(filt)) for measuring the current(i₁) through the second coil (Tr_(filt)), which sensor passes on themeasured data to the control circuit (14), which control circuit (14)controls the switching elements (T₁, T₂) in dependence on themeasurement results of the current sensor (13).
 11. An electroniccircuit as claimed in claim 10, characterized in that the controlcircuit (14) comprises: a first frequency generator (211) for providingcomplementary pulses for an ignition operation of the electroniccircuit, a second frequency generator (221) for providing trigger pulsesfor a normal operation of the electronic circuit, a waveform generator(224) for providing a current reference signal and a lamp currentdirection in accordance with a desired lamp current gradient, acomparator (223) for comparing the measurement results of the currentsensor (13) with the current reference signal from the waveformgenerator (224), such that the output of the comparator (223) isinverted in the case of a desired positive lamp current, and a flipflop(222) with two complementary outputs (Q, /Q) which is set by the triggerpulses of the second frequency generator (221), which is reset by ahigh-level output signal of the comparator (223), possibly after aninversion, and whose complementary output signals are inverted in thecase of a desired positive lamp current, and a process controller (202)for switching over between an ignition operation and a normal operation,which controller for the purpose of normal operation supplies one ofthe—possibly inverted—complementary output signals of the flipflop (222)to one of the switching elements (T₁, T₂) of the half bridge so as tocontrol the latter, such that the switching element (T₂) at the side ofthe reference potential is switched on and the switching element (T₁) atthe side of the operating voltage is switched off the moment the secondfrequency generator (221) generates a trigger pulse if a positive lampcurrent is desired, and the switching element (T₂) at the referencepotential side is switched off and the switching element (T₁) at theoperating voltage side is switched on when the measured value of thecurrent sensor (13) undershoots the reference value of the waveformgenerator (224), whereas for a desired negative lamp current theswitching element (T₁) at the operating voltage side is switched on andthe switching element (T₂) at the reference potential side is switchedoff the moment the second frequency generator (221) generates a triggerpulse, and the switching element (T₁) at the operating voltage side isswitched off and the switching element (T₂) at the reference potentialside is switched on when the measured value of the current sensor (13)overshoots the reference value of the waveform generator (224).
 12. Anelectronic circuit as claimed in claim 11, characterized in that thefirst frequency generator (211) makes available the complementary pulsesat a frequency which corresponds to the resonance frequency of theserial tuned circuit comprising the first coil (L_(ign)) and the firstcapacitor (C_(ign)), and in that the second frequency generator (221)for the purpose of a normal operation of the electronic circuit makesavailable the trigger pulses with a frequency which corresponds to thefrequency at which the arrangement of the second coil (Tr_(filt)) andthe fourth capacitor (C_(filt)) forms a blocking filter for the centraltap of the second coil (Tr_(filt)).
 13. An electronic circuit as claimedin claim 11, characterized in that the process controller (202) feedsthe complementary pulses provided by the first frequency generator (211)to the switching elements (T₁, T₂) of the half bridge to control thelatter during an ignition phase.
 14. An electronic circuit as claimed inclaim 13, characterized in that the process controller (202) after theend of the ignition phase switches over the frequency of the triggerpulses provided by the second frequency generator (221) for a short timeto a frequency below the frequency at which the arrangement of thesecond coil (Tr_(filt)) and the fourth capacitor (C_(filt)) forms ablocking filter for the central tap of the second coil (Tr_(filt)). 15.A method of operating a high-pressure lamp by means of an electroniccircuit as claimed in claim 1, characterized in that the switchingelements (T₁, T₂) of the half bridge are controlled such that asubstantially zero-voltage switching takes place each time.
 16. A methodof operating a high-pressure lamp by means of an electronic circuit asclaimed in claim 1, characterized in that, for the purpose of igniting ahigh-pressure discharge lamp (12) connected between the connectionterminals for a high-pressure discharge lamp, the switching elements(T₁, T₂) of the half bridge are switched during an ignition phasesubstantially exactly at the resonance frequency or an odd multiple ofthe resonance frequency of the resonant circuit consisting of the firstcoil (L_(ign)) and the fourth capacitor (C_(ign)).
 17. A lighting systemcomprising an electronic circuit as claimed in claim 1 and ahigh-pressure gas discharge lamp (12) which is connected between the twoconnection terminals of the electronic circuit designed for ahigh-pressure discharge arc lamp.
 18. A device for the display of stillor moving images utilizing an electronic circuit as claimed in claim 1.